The term “Metallocene” as used herein refers to a derivative of cyclopentadienylidene which is a metal derivative containing at least one cyclopentadienyl component which is bonded to a transition metal. The transition metal is selected from Groups IVB, VB, and VIB, preferably IVB and VIB. Examples include titanium, zirconium, hafnium, chromium, and vanadium. A number of metallocenes have been found to be useful for the polymerization of olefins. Generally, the more preferred catalysts are metallocenes of Zr, Hf, or Ti.
Generally, in order to obtain the highest activity from metallocene catalysts, it has been necessary to use them with an organoaluminoxane cocatalyst, such as methylaluminoxane. This resulting catalyst system is generally referred to as a homogenous catalyst system since at least part of the metallocene or the organoaluminoxane is in solution in the polymerization media. These homogenous catalysts systems have the disadvantage that when they are used under slurry polymerization conditions, they produce polymer which sticks to reactor walls during the polymerization process and/or polymer having small particle size and low bulk density which limits the commercial utility.
Some attempts to overcome the disadvantages of the homogenous metallocene catalyst systems are disclosed in U.S. Pat. Nos. 5,240,894; 4,871,705; and 5,106,804. Typically, these procedures have involved the prepolymerization of the metallocene aluminoxane catalyst system either in the presence of or in the absence of a support. An evaluation of these techniques has revealed that there is still room for improvement, particularly when the catalyst is one which is to be used in a slurry type polymerization where the object is to produce a slurry of insoluble particles of the end product polymer rather than a solution of polymer which could result in fouling of the reactor. In the operation of a slurry polymerization in a continuous loop reactor it is extremely important for efficient operations to limit polymer fouling of the internal surfaces of the reactor. The term “fouling” as used herein refers to polymer buildup on the surfaces inside the reactor.
An improved type of solid metallocene catalyst composition that can be used in a slurry polymerization process was revealed in U.S. Pat. No. 5,498,581, the disclosure of which is incorporated herein by reference. That catalyst composition was prepared by combining a cocatalyst with a metallocene that had an olefinically unsaturated substituent, subjecting that mixture to prepolymerization with an olefin in the presence of a liquid to produce a solid prepolymerized catalyst, and separating the resulting prepolymerized catalyst from the liquid and the components dissolved in the liquid. Some specific variations of producing such catalysts are disclosed in WO 99/29738 and WO 98/52686, the disclosures of which are also incorporated herein by reference.
An object of the present invention is to provide yet further improvements for the making of solid catalyst systems of the type disclosed in U.S. Pat. No. 5,498,581. In accordance with another aspect of the present invention, there is provided a method for polymerizing olefins using the new improved version of such solid prepolymerized metallocene catalyst systems.